a treadmill!! and here is why...
for a 135-pound person
running a 10-minute mile pace burns 589 calories
same person riding a bike at a 12mph pace burns 483
while the bicycling is only 100 calories less that 100 calories burned on the treadmill could add up to 10 LBS in 1 year!!

I know the definitive answer to this question. I have a BodyBugg and it measure how many calories I burn while exercising. I have a treadmill and a stationary bike.
I burn more calories on the treadmill than I do on the bike. I believe that this is due to the fact that on the treadmill I am bearing my weight and that burns more calories. On the bike I am seated and that burns less. I can also stay on the treadmill a lot longer than on the bike. The treadmill is the better choice for starting out with exercise for weight loss.

Better yet, ask to walk a neighbor's dog (especially if you live near an older person who can't get out as much). Then you get outside and get to meet people, do the cardio thing, and make a pup happy and fitter all at the same time!
On top of that, it's free!
(if you need a reason to borrow the dog say, " I want to start walking for exercise, but I would feel a lot safer if I have a dog with me".)

I would say a treadmill because you burn more calories by using many different parts of your body. I feel a bicycle is good for your heart and strenght leg muscles only. Of course, diet with these exercises helps. Good Luck.

I would buy a treadmill bc it works mostly your legs but somewhat the rest of your body. The bike only works your legs. So if you want to work out more of your body buy the treadmill.

i would recommend a good quality trampoline. foldable is convenient too. check the benefit of the tramp on the net.

threadmill! exercise bike only helps your legs more.

Threadmill and ride a bike on it. Two in one!

treadmill I prefer tbh